Seismic exploration involves surveying subterranean geological media for hydrocarbon deposits. A survey typically involves deploying seismic sources and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological medium creating pressure changes and vibrations. Variations in physical properties of the geological medium give rise to changes in certain properties of the seismic waves, such as their direction of propagation and other properties.
Portions of the seismic waves reach the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy one type of sensor or both. In response to the detected seismic waves, the sensors generate corresponding electrical signals, known as traces, and record them in storage media as seismic data. Seismic data will include a plurality of “shots” (individual instances of the seismic source being activated), each of which are associated with a plurality of traces recorded at the plurality of sensors.
Seismic data is processed to create seismic images that can be interpreted to identify subsurface geologic features including hydrocarbon deposits. Seismic imaging methods such as migration (e.g., Kirchhoff migration, least-squares migration, reverse time migration, Gaussian beam migration) generally require an earth model as input. The earth model may be a 2-D or 3-D representation of physical properties of the subsurface, including but not limited to at least one of P-wave velocity (Vp), shear-wave velocity (Vs), density (ρ), and anisotropy (e.g., γ, δ, ε). In some instances, these properties will be divided into distinctive formations such as layers and/or geobodies, which may include salt bodies. The layers and/or geobodies may be delineated by surfaces and points along each surface may have an associated implicit seismic traveltime, which is dependent on the physical properties of the subsurface and the geometry of the surfaces. These physical properties may also be used in forward modeling or demigration to obtain seismic travel times associated with each surface in the earth model. When represented in a computer, the earth model may be composed of a set of triangulated surfaces which divides space into a set of regions. Each region is completed surrounded by surfaces, making each region “air tight” so to speak, aside from the outermost “universe” region. Each region is assigned a seismic velocity function, VR(r), where r is a 3-D cartesian vector, with each VR(r) being a cartesian grid, a constant function or a parametric function. These models provide a way to represent a velocity function, VM(r) for use in seismic migration. This function is evaluated as a two step process: 1) Determine which region contains r and 2) evaluate the VR(r) associated with the region. If the boundaries/surfaces of these formations in the earth model do not accurately match the physical boundaries in the real subsurface that the seismic data was recorded from, the seismic imaging will result in a poor image with problems such as, but not limited to, seismic horizons being located out of position and poor focusing of the seismic events. The poor seismic images make proper interpretation of the subsurface difficult.
The ability to define the location of surfaces in the subsurface is crucial to our ability to make the most appropriate choices for purchasing materials, operating safely, and successfully completing projects. Project cost is dependent upon accurate prediction of the position of physical boundaries within the Earth. Decisions include, but are not limited to, budgetary planning, obtaining mineral and lease rights, signing well commitments, permitting rig locations, designing well paths and drilling strategy, preventing subsurface integrity issues by planning proper casing and cementation strategies, and selecting and purchasing appropriate completion and production equipment.
Conventional methods for updating the earth model to show more accurate surfaces are either accurate but computationally expensive or computationally inexpensive but not accurate. For example, known methods include:
Method A: Remove all surfaces below the uppermost region containing a change in VR(r), then one-by-one from top to bottom apply: 1) 3-D seismic migration of the data originally used to develop the original model, 2) reinterpretation of the surface at the base of the region and 3) construction of a model. These three steps are repeated until all needed surfaces are added to the model. This approach is extremely slow and computationally expensive, requiring often many months of work, but potentially yields the most accurate solution.Method B: Use normal ray map demigration with the original model and then iteratively apply from top to bottom: 1) exploding reflector ray map migration and 2) construction of a model. This model is then used in the next iteration for raytracing. This approach is more rapid that method A, but suffers from stability issues with the raytracing used in the map demigration and migration: wild rays are possible resulting in mangled surfaces and shadow zones may yield demigration/migration impossible in certain areas.Method C: Use vertical time invariance rather than normal ray time invariance. Stability is high, and the method can be implemented in a manner such that the original model is simply warped into the updated model, eliminating the model reconstruction step. The accuracy can be poor, however, for models with significantly dipping surfaces or substantial lateral velocity changes, such as models containing salt bodies.
There exists a need for a fast, accurate method of updating earth models that allow for improved subsurface interpretation.